Recently, attention has been drawn to a boron neutron capture therapy (BNCT) as a novel cancer therapeutic method utilizing a radioisotope. The boron neutron capture therapy is a therapeutic method in which a boron compound containing boron-10 isotope (10B) is delivered to cancer cells and the cancer cells are irradiated with a low energy neutron (for example, thermal neutrons), and thus the cancer cells are locally destroyed by a nuclear reaction which arises in the cells. In this therapeutic method, since it is important to cause boron compound(s) which contain 10B to be selectively accumulated by cells of cancerous tissue so as to enhance therapeutic effect, boron compounds which are selectively uptaken by cancer cells have been developed.
Hitherto, some boron-containing compounds in which boron atoms or boron atomic groups are introduced into a basic skeleton have been synthesized as an agent used in BNCT. Examples of such an agent used in the actual clinical practice include p-boronophenylalanine (BPA) and mercaptoundecahydro-dodecaborate (BSH: borocaptate). Among these agents, BSH is a boron cage (cluster) compound having the lowest toxicity, which is mainly used in the form of a sodium salt for the treatment of brain tumor, and thus utility thereof has been confirmed (see, for example, Non-Patent Documents 1 to 8).